e-Terra Geochemical Characterization of Holocene

e-Terra
http://e-terra.geopor.pt
ISSN 1645-0388
Volume 5 – nº 10
2008
Revista Electrónica de Ciências da Terra
Geosciences On-line Journal
GEOTIC – Sociedade Geológica de Portugal
_________________________________________________________
Geochemical Characterization of Holocene Sediments of Santo
André Alluvial Plains (SW Portugal)
SANDRA C. C. MOREIRA – sandraccmoreira@gmail.com (Universidade de Lisboa, Faculdade de Ciências,
Departamento e Centro de Geologia, ed. C6, 3º piso, Campo Grande, 1749-016 Lisboa, Portugal)
MARIA C. P. FREITAS – cfreitas@fc.ul.pt (same address as S.C.C. Moreira)
MARIA F. ARAÚJO – faraujo@itn.pt (Instituto Tecnológico e Nuclear, Departamento de Química, Grupo de
Química Analítica e Ambiente, Estrada Nacional 10, 2686-953, Sacavém, Portugal)
CÉSAR ANDRADE – candradre@fc.ul.pt (same address as S.C.C. Moreira)
ANABELA G. CRUCES – acruces@fc.ul.pt (same address as S.C.C. Moreira)
ABSTRACT: A paleoenvironmental reconstruction of Santo André lagoon area throughout the Holocene is
presented based upon sedimentological and geochemical study of two cores – “Cerradinha 14” and
“LSA”, which were taken from the infill of adjacent alluvial plains.
The results of textural analysis (coarse fraction), composition (organic matter) and geochemistry (major,
minor and trace elements – Si, Al, Fe, Ca and Cl) allowed the identification of four major
lithostratigraphic units, present in both cores, corresponding to a time succession of distinct sedimentary
environments – marine, lagoonal and fluvial - in the last 10000 years.
KEYWORDS: Holocene evolution, paleoenvironmental reconstruction, sedimentary record, geochemistry,
coastal lagoon.
1. INTRODUCTION
The lagoons of the SW Portuguese coast were formed about 5 000-5 500 BP, during the
Holocene transgression, when the deceleration of sea level rise and relative stabilization of sea
level allowed the formation of detrital barriers, thus protecting coastal water bodies from the
open ocean (Freitas et al., 2002).
Since lagoons work as sediment receptors these have been filled since then, creating a
continuous sedimentary record with the ability to register hydrodynamic and ecological
variations, allowing a paleoenvironmental reconstruction of these areas.
Santo André is the largest lagoonal system in the SW Portuguese coast, located in the
southern half of Tróia – Sines embayment, 80km away from Lisbon. This lagoon has a 4km long
sandy welded barrier extending between Santo André and Monte Velho beaches.
Its drainage system is constituted by five main tributaries, which drain about 142 km2 (fig. 1),
producing ≈12 000-30 000 ton/year of sediment (Cruces, 2001). These are, from N to S,
Cascalheira, Ponte, Forneco, Azinhal and Badoca rivulets. It is possible to define two major subbasins, since water and transported sediments are flushed into the lagoon through:
- the Cascalheira system, in the north, draining 31.5 km2;
- all other tributaries, which converge in a single channel just before outleting in the
southern region of the lagoon.
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Figure 1 – Drainage network outleting in Santo André lagoon (adapted from Cruces, 2001) and location of studied
cores. The blue line separates major sub-basins.
Both networks develop in their downstream sections extensive alluvial plains, which are
designated in this work by Cascalheira alluvial plain (CAP) and Southern tributaries alluvial
plain (STAP).
The rocks outcroping in Santo André watershed range in age between Paleozoic and Holocene
(fig. 2), and occur in bands elongated N-S associated to a westward dipping monocline structure
(Cruces, 2001). The Paleozoic formations (31% of the outcrops) belong to South Portuguese
Zone and their ages range from Devonian to Carboníferous. They essentially consist of
greywacke and slate turbidites and are represented here by Mértola and Mira Formations.
Mesozoic lithologies (16% of the outcrops) are essentially sandstones, marls, dolomites and
limestones from Triassic to Jurassic ages. Cenozoic materials occur in 50% of the drainage area,
specially in its western region and consist of detrital and permeable rocks (siltstones to
sandstones). They are mostly from the Plio-Plistocene, but also include punctual Miocene
occurrences and Holocene materials (dune and beach sand and alluvium).
In order to compare the holocenic infill of both alluvial plains, two cores were studied:
- “Cerradinha 14”, representative of CAP infill until 8,44 m below surface, collected in
May 2004 with a Van der Horst sampler; and
- “LSA”, collected in June 1998 with a Shelby sampler in STAP, reaching the Miocene
basement 25 m below the surface.
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Figure 2 – Geology of Santo André watershed (adapted from Carta Geológica de Portugal, scale 1/200.000, sheet 7,
S.G.O., 1993 and Carta Militar de Portugal, scale 1/25.000, sheet 505, IGeoE, 2001). The black line separates major
sub-basins.
In this paper, the results of sedimentological and geochemical analyses of these two cores are
presented. This allowed the definition of lithological units and, together with other indicators, the
establishment of a paleoenvironmental evolution model.
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2. METHODOLOGY
Sediment samples from both cores were submitted to 63 µm wet sieving to determine
proportions of coarse/fine fractions. Organic matter (O.M.) content was determinate by titration,
following oxidation (LNEC, 1967).
Different methods were used in each core for the geochemical study (major, minor and trace
elements – Si, Al, Fe, Ca and Cl). “Cerradinha 14” samples were analyzed by energy dispersive
X-ray fluorescence spectrometry at the ITN (Sacavém, Portugal); “LSA” sediments were studied
by XRF – X-ray Fluorescence Spectroscopy (major and minor elements) and INAA –
Instrumental Neutron Activation Analysis (trace elements) at ACTLABS (Canada). In all cases,
geochemical analyses were performed on total sediment.
3. RESULTS AND DISCUSSION
The sedimentological and geochemical study of the two cores, complemented by unpublished
paleoecological data and radiocarbon dating, allowed the identification of four lithostratigraphic
units and the reconstruction of respective sedimentation environments for both alluvial plains
(tables 1 and 2).
Depth
(below
surface - m)
0.00 – 3.37
3.37 – 4.05
IV
III
Bottom of
unit
(14C age)
-
4.05 – 8.39
II
7220 BP*
8.39 – 8.44
I
-
Unit
Sedimentary
Environment
Description
Muds, with pulmonate gastropods.
Slightly muddy to muddy sand, shells absent.
Alternation of peat with organic mud containing shells
fragments of brackish/marine molluscs and occasional
shell rich laminae.
Fluvial
Marine/Lagoonal
Muddy sand, shells absent.
* interpolated age estimated using lithostratographic correlation.
Table 1 – Lithostratigraphic succession of “Cerradinha 14” sedimentary column (adapted from Moreira, 2006).
Depth
(below surface - m)
1.00 – 2.00
2.00 – 2.35
2.35 – 5.22
5.22 – 11.00
11.00 – 14.30
Unit
IV
III
II
14.30 – 17.00
17.00 – 20.80
20.80 – 25.00
25.00 – 25.45
I
Basement
Subunit
B
A
Bottom of unit
( 14C age)
c. 350 BP*
1620 ± 40 BP
B
3570 ± 50 BP
A
5380 ± 50 BP
C
10020 ± 50 BP
B
A
-
12440 ± 60 BP
14160 ± 60 BP
-
Description
Mud with rare sand laminae.
Mud
Sand with rare mud laminae.
Mud; few organic levels and abundant
bioclasts.
Gravel at the bottom; alternation of
sand, muddy sand and mud, with
predominance of the former.
Fine sand and stiff mud.
Sedimentary
Environment
Fluvial
Lagoonal
Marine
Fluvial
-
Table 2 – Lithostratigraphic succession of “LSA” sedimentary column (* interpolated age) (adapted from Freitas et
al., 2003).
3.1 Sedimentological characterization
The sedimentation pattern is similar in both alluvial plains (fig. 3). Coarse fraction (>63µm) is
present in high proportions (>25%) in basal units (I and IC) reducing towards unit II in both
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cores, where these percentages are normally lower than 25%. In unit III, constituted by sand, the
coarse fraction is higher than 50%, and decreases again towards unit IV which is mainly
constituted by mud.
Figure 3 – Vertical variation of coarse fraction (>63µm) and organic matter content. The first meter in “LSA” core
was not sampled because it corresponds to a landfill.
Vertical distribution of organic matter is very similar along both sedimentary columns, the
maximum occurring in unit II. However, in “LSA” O.M. never exceeds 25%, whereas in
“Cerradinha 14” the organic content, in general, exceeds this value due to the presence of peat
and very organic muds. Peats have not been found in the STAP, and its existence in the CAP
could be associated to a marginal, low hydrodynamic environment, associate to a small water
column, favouring colonization by vegetation.
3.2 Geochemical characterization
In units I and III of “Cerradinha 14”, constitute by slightly muddy sands and muddy sands,
both essentially quartziferous, Si increase is accompanied by a decrease of Al and Fe
concentrations. In units II and IV (mud and peat) these elements present similar behaviour
increasing and decreasing in phase (fig. 4). In “LSA” both elements Al and Fe generally show a
behaviours opposite to Si (fig. 4), which is confirmed by strong negative correlations (r2=0.85
and 0.93, respectively). The signal of the correlation depends on sediment composition: in
essentially minerogenic sediments, correlation is negative; however, in organic fine sediments
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(peat and organic mud levels) the high percentage of organic matter induces a simultaneous
decrease of the major elements - Si, Al and Fe - revealing positive correlation.
Figure 4 – Vertical variation of silicium (Si), aluminium (Al) and iron (Fe). The first meter in “LSA” core was not
sampled because it corresponds to a landfill.
The Ca content in both cores is higher in unit II (fig. 5), mainly due to the existence of shells,
shell debris and bioclastic levels (mainly marine and brackish bivalves), with maximum
concentrations of 3% and 6% at “Cerradinha 14” and “LSA”, respectively. In other units the
concentration values of this element are generally lower, except in the superficial samples,
because of the presence of fresh water gastropod shells.
In both cases the Cl vertical variation (fig. 5), which we use as a paleosalinity indicator, limits
a sedimentary package including units I and II, with higher Cl concentrations. Comparatively, Cl
concentrations are higher in “LSA” than in “Cerradinha 14”, with maximum values of 0.44%
and 0.28%, respectively. Units III and IV exhibit lower values.
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Figure 5 – Vertical variation of calcium (Ca) and chlorine (Cl). The first meter in “LSA” core was not sampled
because it corresponds to a landfill.
4. INTERPRETATION
Dating information suggests that “Cerradinha 14” core is temporally equivalent to IC, II, III
and IV units of “LSA” allowing the comparison of sedimentary packages according to their
textural and geochemical characteristics. Given that the thickness of “LSA” sedimentary column
is approximately twice of “Cerradinha 14” within the same time interval, sedimentation rates
were necessarily higher in the former case.
Three sedimentary environments were identified in “LSA” sedimentary column (Table 2): 1)
MARINE (OPEN ESTUARY/RIA), between 10 020 e 5 380 BP, corresponding to the Holocene
transgression associated to a high sea level rise rate; 2) LAGOONAL, between 5 380 and 1 620
BP, following the formation of a sandy barrier contemporary to the slowing down of the sea
level rise rate and stabilization of mean sea level; and 3) FLUVIAL, after 1 620 BP,
corresponding to the progradation of alluvial fans which invaded the prior lagoonal space.
“Cerradinha 14” core seems to show less evidence of marine influence in comparison with
“LSA”. Preliminary paleoecological data obtained from foraminifera and nannoplankton (Maria
Alday and Maria de Jesus Ramalho, personal communication) confirm the existence of a
brackish lagoonal environment since circa 7 300BP (units I and II), with maximum marine
influence at 7.29m. In fact, paleoecological indicators of a fully marine environment have not
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been found and this may be a consequence of the shelter effect of the Cerradinha resistance relief
at Cascalheira depression. This environment evolved to freshwater fluvial conditions (unit III
and IV) in a similar way to the one pointed for “LSA”. Units III and IV are interpreted as a shift
from the alluvial front (coarser sediments, high energy) to alluvial plain deposition (lower
energy).
5. CONCLUSIONS
The evolution of alluvial plains contiguous to Santo André lagoon throughout the Holocene
was similar. Comparative sedimentological and geochemical analyses of “Cerradinha 14” and
“LSA” columns allowed the identification of two distinct sedimentary packages, representative
of an environment with clear marine influence, followed by a shift to fresh water fluvial
conditions. Sedimentological and geochemical differences between the two sedimentary columns
are essentially associated to local factors. “Cerradinha 14” core presents a more organic
sedimentation with lower Ca and Cl concentrations comparatively to “LSA”, probably due to
marginal, low energy local conditions and sheltering conferred by the Cerradinha resistance
relief.
Acknowledgements
This paper is a contribution of FCT Project POCTI/MAR/15231/99 and was partially supported by the
grant PRAXIS XXI/BD/21564/99.
References
Cruces, A.G. (2001) – Estudo a micro e meso-escala temporal de sistemas lagunares do SW
alentejano (Portugal): As lagunas de Melides e Santo André. Tese Mestrado, GeoFCUL,
228 p.
Freitas, M.C., Andrade, C., Rocha, F., Tassinari, C., Munhá, J.M., Cruces, A., Vidinha, J. &
Silva, C.M. (2003) – Lateglacial and Holocene environmental changes in Portuguese coastal
lagoons 1: the sedimentological and geochemical records of the Santo André coastal area.
The Holocene 13, 435-448.
L.N.E.C. – Laboratório Nacional de Engenharia Civil (1967) – Especificação
E201, Solos –
Determinação do teor em matéria orgânica. Documentação normativa, Outubro de 1967. 3
p.
Moreira, S.C.C. (2006) – Assinatura Geoquímca de Sedimentos Estuarinos e Lagunares do SW
Alentejano – Lagoa de Santo André. Relatório de Estágio. GeoFCUL, 140 p.
Presented during the VI Congresso Ibérico de Geoquímica / XV Semana de Geoquímica, Universidade de Trás-osMontes e Alto Douro, Vila Real, Portugal, 16-21 July 2007.
Received 21 January 2008
Revised 25 January 2008
Published 6 May 2008
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